In many applications, it is important to drive an element of a machine along a lead screw with accurate positional repeatability and constant drag torque in both forward and reverse directions. Data printers and x-y tables, used as peripheral equipment in the computer industry, for example, have such requirements.
Positioning devices designed to meet these requirements have been proposed, and many of these employ an anti-backlash nut to achieve the positional accuracy along the screw which is required. Examples of two such anti-backlash nut assemblies which have been proposed are described in the patent literature as follows.
In U.S. Pat. No. 3,656,358, issued to Kopp, a linear positioning device is disclosed which is stated to have an improved collar for use with a comparatively inexpensive rod having multiple grooves. The collar is telescoped over and adapted to be translated back and forth relative to the elongated rod. This collar includes cantilevered fingers which are resiliently wedged into angularly spaced grooves formed in the rod to preload the collar onto the rod and prevent rotational play from developing between the two. In a specific embodiment, the collar is telescoped onto a rod in the form of a splined shaft while in another embodiment, the collar is a nut threaded onto a screw with multiple threads.
In U.S. Pat. No. 3,997,269, issued to Linley, an anti-backlash, self-aligning nut construction with specially constructed tubular nut bodies which co-act with concentric spring sleeves is described. The nut bodies, in general, each have a pair of spring-biased elements provided with internal thread formations adapted for engagement with the external threads of a screw. In one embodiment, a self-aligning spring sleeve is provided having solely three pairs of oppositely-disposed transverse slots to obtain the desired aligning features. The nut body has a base portion which is separated from the spring-biased elements by means of two transverse slots which, together with an adjacent pair of slots in the spring sleeve, form in effect a universal joint. One of the remaining slot pairs in the sleeve is oriented circumferentially with respect to the first pair by an angle of 90.degree., with a third pair of slots being circumferentially aligned with the first pair.
More recently, an anti-backlash nut having oppositely-directed longitudinal flexure members has been disclosed in U.S. Pat. No. 4,210,033, which issued to the present inventors. This anti-backlash nut has a continuous portion extending longitudinally from one end of the screw to the other. In addition, there are at least two, and usually more, oppositely-directed longitudinal flexure members which have one end fixed to the anti-backlash nut and one end free-floating. The oppositely-directed longitudinal flexure members are biased towards the screw by one or more radial springs or other means for biasing.
One style of anti-backlash nut has one or more longitudinal flexure members with one end of each member fixed and one end free floating in cantilever fashion. The nut, including the flexure members, undergoes translational movement along a threaded shaft or lead screw. Each longitudinal flexure member has a ramp at its free-floating end. The nut is surrounded by an annular pressure applying ring which derives its force from a compression spring. The ring is constantly urged against the ramps which, in turn, creates radial force vectors to maintain the internal threads formed on the flexure members in contact with the threads of the shaft during operation and even after the nut has become worn. Such an anti-backlash nut is disclosed in our U.S. Pat. No. 4,249,426 which reissued as RE. 32,433.
The threads on the shaft are in the form of a helix, as are the mating threads on the interior of the flexure members in the anti-backlash nut described in U.S. Pat. No. 4,249,426. When a load is placed on the nut, as for example, when it is attached to a carriage or printer, there is a substantial force component acting axially on the shaft or lead screw and bearing on the threads of the flexure members. The axial force translates into two force vectors, one in the axial direction of the shaft and the other normal thereto, tangential to the shaft. This induces the cantilever mounted longitudinal flexure to deflect in a direction normal or tangential to the axis of the shaft. This can induce unwanted backlash.
U.S. Pat. No. 5,027,671 discloses an anti-backlash nut with an annular member to counteract the unwanted backlash To add structural rigidity to the flexure members and to counteract force acting in a direction which is tangential to the longitudinal direction of movement of the nut, cooperating spline are provided on the longitudinal flexure members and on the annular member. The annular member surrounds the nut body to apply an external force radially inwardly to each of the ramps.
The spline includes a longitudinal groove in each of the flexure members extending parallel to the axis of the screw and a complementary mating longitudinal ridge projecting from the annular member engageable within the groove.
Whenever a load is placed on the nut, thread flanks tend to wear. Since the flanks are angularly disposed to the axis of rotation of the screw, upon wear the threads on the fingers drop more deeply into the threads of the screw and vice versa, until crests abut roots and flanks separate. The wear of the flanks thus contributes to axial play. To operate correctly and assure no backlash, both flanks of a thread must engage the flanks of adjacent threads on the mating part.
It is, therefore, an object of the present invention to provide an anti-backlash nut having longitudinal flexure members in which the flexure members are tailored to minimize radial play and achieve a radial and axial stiffness even as surfaces wear.